Organometallic compounds are compounds having a carbon-metal bond (the compounds can include hydride compounds having a hydrogen-metal bond). In general, the carbon-metal bond has high reactivity, and the reactivity is various depending on types of organic groups and metals. Thus, the organometallic compounds have become reaction reagents indispensable to current chemical industry, such as for the use in various organic synthesis reactions. A binuclear organometallic compound having plural carbon-metal bonds in one molecule is expected to have properties such as more various reactivity. Although several cases have been reported about a binuclear organometallic compound having no functional group (Non-Patent Documents 1, 2, and 3), a synthesis method thereof can be hardly said to be an industrial synthesis method. Furthermore, no cases have been reported about a binuclear organometallic compound having a functional group.
On the other hand, it has been well known that in the production of polymers such as polyethylene and polypropylene using a transition metal polymerization catalyst, organometallic compounds such as diethylzinc, trimethylaluminum, triethylaluminum and trioctylaluminum function as a molecular weight regulating agent. Gibson et al. reports that the use of dialkylzinc compounds in olefin polymerization using a certain kind of a transition metal complex can provide an ethylene oligomer one terminal of which is capped with a metal wherein the molecular weight distribution is Poisson-like distribution (Patent Document 1). Here, Poisson distribution is represented by χp=(xpe−χ)/p!, wherein χp is the mole fraction of a polymer obtained after p number of olefins are polymerized, and x is the Poisson coefficient. Because the molecular weight distribution is Poisson-like distribution, the chain transfer from the catalyst to the metal is considered to be reversible, and a chain transfer agent with such nature is referred to as a reversible chain transfer agent. When the reaction of the chain transfer to the organometallic compound is reversible and as fast as chain growth reaction, no unreacted organometallic compound remains and a polymer one terminal of which is capped with a metal can be efficiently obtained.
The reaction of the chain transfer to the organometallic compound in olefin polymerization utilizes a metal exchange reaction, one of the elementary reactions in organometallic chemistry, thereby obtaining a polymer in which formally, the organic group derived from the organometallic compound employed as an molecular weight regulating agent is bonded to one terminal of the olefin polymer, and the metal derived from the organometallic compound is bonded to the other terminal. Modifying a carbon-metal bond at the terminal can lead to the introduction of a functional group into the terminal of the polyolefin.
Furthermore, a both-terminal-functional olefin polymer, having a functional group at both terminals, can be provided with more various properties. Such a both-terminal-functional olefin polymer is synthesized by living polymerization, hydrogenation of metathesis polymers, thermal decomposition of polyolefins, but the synthesis is accompanied by low productivity, low functionalization ratio, low molecular weight, and the like, which limits the application scope.
Among the organometallic compounds, an organozinc compound has been attracting attention for its ability to coexist with functional groups due to its moderate reactivity. Thus, the organozinc compound has been intensively developed as a multi-functionalized organometallic reagent.
The organozinc compound is usually prepared from zinc halides, and organic lithiums or Grignard reagent, the preparation involving by-production of inorganic salts such as lithium halides and magnesium halides. Such by-produced inorganic salts possibly have an influence on reactivity, and thus need to be removed. As a method to remove the by-produced inorganic salts, it has been reported by Charette et al. that a reaction mixture is subjected to centrifugation and a supernatant is used to thereby prepare an organozinc reagent free of by-produced inorganic salts (Patent Document 2), but it must be noted that the centrifugation method under inert atmosphere is difficult to employ on an industrial scale.
A method involving no generation of such inorganic salts has been reported, and effective means is boron-zinc exchange reaction. In this reaction, hydroboronation of borane with a terminal alkyne or a terminal alkene produces an alkyl borane or an alkenyl borane, to which dimethyl or diethyl zinc is acted, and thereby an alkyl or alkenyl zinc compound in which boron has been exchanged with zinc can be obtained. Because the by-product is trialkylborane, which has a low boiling point, the by-product is easy to remove under reduced pressure at a laboratory level, and various cases have been reported about the preparation of various multifunctional alkylzinc compounds (Non-Patent Document 2). In addition, several cases have been reported about the synthesis of cyclic organozinc compounds both terminals of which are metallized by similar methods (Non-Patent Documents 1 and 3). As another method involving no generation of inorganic salts, halogen-metal exchange method can be mentioned. Knochel et al. have reported that an alkyl iodide compound having a functional group is acted to diethylzinc, and then an excessive of diethylzinc and ethyl iodide are removed under reduced pressure, thereby synthesizing an alkyl zinc compound having a functional group (Non-Patent Document 4).
These synthesis methods involving no generation of the inorganic salts, however, require an excessive amount of diethylzinc accompanied by no solvent in order to obtain high reaction yield. In addition, instead of no generation of inorganic salts, alkylboranes, ethyl halides, and the like are by-produced. For this reason, such a method on an industrial scale needs to be performed with advanced reaction control and with advanced safety measures about the use of diethylzinc accompanied by no solvent. Furthermore, it is necessary to provide equipment for removing low boiling point compounds including diethylzinc.
As described above, a number of cases have been reported about methods to synthesize organometallic compounds such as organozinc compounds. However, none of the methods can be said to be efficient in terms of atom economy, because by-products are generated such as inorganic salts, alkylboranes and alkyl halides, and there has been no method provided which are employable and efficient on an industrial basis. Therefore, a more practical production method has been desired.